A packet switching system, such as may be used in telephony, is generally made up of a plurality of packet switching nodes interconnected by communication links such as telephone trunks. A packet switching node commonly comprises a packet switching network for selectively connecting various of its input ports to various of its output ports; link controllers each for interfacing a communication link to an input port and an output port of the packet switching network; and a central processor for controlling the operation of the switching node. Such packet switching nodes and systems are known in the art. An example thereof is disclosed in the copending U.S. patent application of Jonathan S. Turner, entitled "An Interface Facility for a Packet Switching System", Ser. No. 392,222, filed on June 25, 1982, issued on Dec. 11, 1984, as U.S. Pat. No. 4,488,289, and assigned to the same assignee as this application.
Packets are commonly transmitted across the links between nodes in packet switching systems only when particular tasks need to be performed in the system or when communication data are available for transmission; otherwise, no packets are transmitted and the links are idle. Tasks that require transmission of packets include call path setup tasks and various maintenance tasks. Data is commonly available for transmission in bursts, such as when the parties to a telephone call are talking.
Often it is undesirable to leave the interconnecting links idle. For example, it is desirable to periodically make a transmission across a link to assure the receiving node that the transmitting node and the interconnecting link have not failed. Also, certain links include repeaters that continuously need to receive transmissions in order to maintain their synchronization with the pair of nodes that are interconnected by the link. For such reasons, certain link communication protocols also require that transmissions across the link be made at least periodically, and they signal link communication errors if the required transmissions are not detected.
For these purposes, the prior art has suggested transmitting an idle code on the links during idle periods when no packets are available for transmission. Such an idle code may take the form of a rest configuration byte, which is a predetermined sequence of bits recognized by the receiving node as idle code. A rest configuration byte may be, for example, a packet start and stop flag. As the name indicates, the flag is a predetermined sequence of bits that signals the beginning or end of a packet.
Continuously repeated transmissions of idle code create certain problems, however. For example, a receiving node cannot determine from receipt of idle code whether the transmitting node is functional or failed and stuck in an idle code-transmitting mode. Certain idle codes, for example those made up of all zeros or all ones, fail to satisfy the synchronization needs of repeaters, because repeaters commonly synchronize their operation to transitions between received bit signals and hence they need to detect transitions between zeros and ones. Furthermore, certain link protocols derive meaning from bit patterns, such as the repeated occurrence of a given bit value in every nth transmitted bit, and these link protocols may find unintended meaning in continuously repeated transmissions of idle code. Also, transmissions on a link of repeating patterns often cause crosstalk on adjacent links, thereby degrading the quality of communications on those links.
In summary, transmissions are desirable or necessary during idle periods on links that interconnect nodes in packet switching networks, but the prior art approach of transmitting a continuous sequence of idle code during idle periods often does not satisfy link requirements, and in certain applications creates additional problems.